1. Field of the Invention
This invention relates to methods for preparing foundry sand and more particularly to a method for automatically controlling the moisture content of foundry sand.
2. Description of the Prior Art
In foundry operations, foundry sand is mixed with water and used for molds and cores which are in turn used in casting operations. Moisture control is known to be an important factor in obtaining durable molds and cores. It is known to measure the moisture content of foundry sand prior to mixing the sand in a mixer in order to calculate the volume of water that should be added to obtain the correct moisture content. An example of such a system is U.S. Pat. No. 4,569,025 which teaches the automatic measurement of sand moisture content along with other foundry sand parameters. The measurement of moisture content can take place in the mixer or as the sand is transported to the mixer.
A system of the latter type is known wherein the moisture content of sand is calculated by measuring the loss of microwave energy through the sand and the temperature of the sand. In this apparatus, a layer of sand is conveyed from a hopper to a mixer along a belt. Microwave energy and infrared temperature measurements are taken as the sand is moved along the belt. Based on these readings and assuming a constant belt speed, a volumetric addition of water is automatically calculated, using an analog circuit, and added to the sand after it enters the hopper. Although the infrared temperature sensors and microwave measurements give precise information on sand temperature and moisture content, these methods suffer from restricted sampling area and therefore often lead to wide deviations between the actual and desired sand moisture content. In addition, the volumetric rate of sand transport often varies from its predicted rate therefore leading to additional variations between the actual moisture content of sand leaving the mixer and the desired moisture content of the sand.